Enabling deep non-existing path for extracted source map

Hi

Thanks for this project. I really don't like the approach of having two separate builds for dev and prod and the inline source map is giving me a hard time (even if I like the idea in general). Also I love the name of you project :smile: 

This pull requests uses mkdirp for creating non-existing directories in the path specified for the source map. I have this use case in my project and therefore would like to propose this pull request. I've also added a test and refactored the test setup method which I renamed to cleanup which is executed on test finish.

Cheers
Gion